This invention relates to coating, and more particularly to methods of slot extrusion coating by first determining a window of operability.
Coatings are generally applied as a uniform, continuous layer. Slot extrusion coating is just one way to coat a composition onto a substrate, as many other methods are available such as coating by curtain, knife or blade, forward-roll, reverse roll, or slide methods. Slot extrusion coating is particularly useful for applying coatings at high substrate speeds and for precision applications. Coating by slot extrusion can provide precise, premetered quantities of a composition. In general, slot extrusion coating is used to deliver thin sheets of material (e.g., coating) onto a substrate by feeding fluid to a coating die, which in turn, then applies the fluid to a substrate in the form of a sheet or film. A coating bead is often used to refer to the bridge of liquid spanning the gap between a die and a substrate.
Many studies have been performed to understand or model the dynamics and other behavioral effects liquid compositions have during coating operations. For example, rheology, shear thinning, viscosity, elasticity, Newtonian or non-Newtonian flow, inertial effects and extensional effects, to name just a few, have been subjects of coating studies. Of particular interest in studying these effects and characteristics is the manageability and optimization of coating methods to achieve coatings less susceptible to drying defects. The coatability of a composition in combination with a particular coating technique is an area of interest, especially for operations that desire thin coatings, use high solids content, or both.
Typically, in premetered coating techniques, the flow rate per unit width of a substrate, in combination with the substrate speed, can determine the thickness of a coating layer or sheet. Advantageously, the premetered coating technique of slot extrusion coating can provide a high precision coating of thin layer by merely prescribing the flow rate of the liquid as it is fed into a coating die, and may be independent of other process variables. Conventional methods prescribed that higher line speeds would require thicker wet layers. Thus, to attain thinner coatings, one skilled in the art generally decreases the flow rate and substrate speed. The ability to decrease flow rate, however, is generally limited by the Theological properties of the coating composition itself. Decreasing a flow rate too low can result in the non-uniform or unstable sheets. Further, reduction in substrate speed is generally undesirable because of resulting reduction in manufacturing productivity.
It is also recognized in slot extrusion that lowering the viscosity of the coating composition is another method used by those skilled in the art to reduce the thickness of the resulting coating. This is accomplished by adjusting the composition or reducing the percent solids of the coating liquid. Lower viscosity layers are often susceptible to undesirable drying patterns, such as mottle or Benard cells, in the finished coating.
It has been attempted to control coating thickness by modifying the size of a coating gap located between a die and a substrate. That is, it was thought that thinner coatings can be achieved with tighter or smaller gaps. However, gaps under 100 microns, for example, can result in operating difficulties, as particulate matter can accumulate in the coating gap and subsequently create defects such as streaks.
The coatability of a composition in combination with a particular coating technique is an area of interest, especially for operations that desire thin coatings, use high solids content, or both. What is desired is a method of slot extrusion coating a substrate using a composition having a high solids content, that can be applied at a reasonable, production-worthy substrate speed to provide high quality coatings. Methods that can provide thin sheets of coating at acceptable substrate speeds would also be desirable.
A method of slot extrusion coating is provided that can be used to apply thin coatings using liquid compositions having high percent solids. A window of operability advantageously identifies the limits of a process to provide these thin high solids coatings.
In a preferred aspect, a method for slot extrusion coating is provided, where the method includes:
providing a liquid composition having at least one polymer and a diluent, where the composition is substantially free of crosslinking and gellation and has a measurable percent solids;
operating a slot extrusion coater wherein said liquid composition is extruded from said slot extrusion coater;
determining actual values of minimum wet thickness, Tw,min at more than one level of percent solids;
obtaining a first graphical curve representing actual values of wet thickness, Tw,min as a function of percent solids level;
identifying the critical wet thickness, Tw,min-critical on the first graphical curve; and
identifying a window of operability as an area defined by the boundaries: percent solids greater than the point at which critical wet thickness, Tw,min-critical occurs; and an actual wet thickness greater than all points above the first graphical curve and equal to or less than the critical thickness, Tw,min-critical 
In another aspect of the invention, the method further includes steps of:
defining a target dry coating weight, WD;
calculating a plurality of values for Tw,calc (in meters) using formula (I), each Tw,calc value corresponding to a percent solids level, wherein formula (I) is
Tw,calc=(100*WD)/(%S*xcfx81L)xe2x80x83xe2x80x83(I)
wherein WD is the dry coating weight (kg/m2), %S is the percent solids, and xcfx81 is coating liquid density (kg/m3);
obtaining a second graphical curve representing calculated values of wet thickness, Tw,calc as a function of percent solids level; and
identifying the window of operability as the area defined by the boundaries:
Tw,calc greater than the first graphical curve and a percent solids level greater than the point at which Tw,min critical occurs.
In a further aspect, a method of the invention includes additional steps of adjusting the liquid composition to have a percent solids within the window of operability; and coating a substrate with the liquid composition at a Tw that falls within the window of operability.
In yet another aspect of the invention, a coated substrate using a method of the invention is provided, where a coating made in accordance with the invention is substantially free of coating instabilities.
As used herein and in the claims, the following terms have the meanings as now set forth:
a xe2x80x9cbeadxe2x80x9d or a xe2x80x9csheetxe2x80x9d is descriptive of the liquid coating that emerges from the coating die;
xe2x80x9coperability windowxe2x80x9d or xe2x80x9cwindow of operabilityxe2x80x9d is the range of certain parameters in which a coating process can operate to provide and maintain a coating bead according to the present invention; and
xe2x80x9cconventional coating techniquesxe2x80x9d include the range of coating parameters that permit the application of a coating onto a substrate that are not within the operability window of the present invention and do not provide the advantageous effects of the present invention.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.